1. Field of the Invention
The present invention relates generally to a hydraulic shock absorber, such as for automotive suspension system. More specifically, the invention relates to a valve construction to be employed in the hydraulic shock absorber for achieving piston stroke speed dependent variable damping characteristics.
2. Description of the Background Art
It should be appreciated that in automotive suspension system a shock absorber is required to successfully absorb vibration energy which causes vibration in the vehicle body in order to provide for riding comfort of the vehicle. On the other hand, the shock absorber has to damp vibration for suppressing relative displacement between the vehicle body and a suspension member so as to suppress attitude change for driving stability.
It should also be appreciated that vibration energy generated in a road wheel due to uneveness generally influences the riding comfort of the vehicle and has relatively small magnitude and high frequency to cause small magnitude and high speed piston stroke in the shock absorber. On the other hand, vibration energy induced in the vehicle body generally influences the driving stability for causing attitude change, such as pitching, rolling and so forth, and has relatively great magnitude and low frequency to cause great magnitude and low speed piston stroke.
In order to provide both of the riding comfort and driving stability, it is therefore required to absorb high frequency vibration and to damp low frequency vibration. In the prior art, there have been proposed various shock absorbers which attempted to provide piston stroke dependent damping characteristics for accomplishing both of the aforementioned tasks.
For example, Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 61-47134 discloses a shock absorber with a multi-stage valve assembly employed in a shock absorber piston. In the shown construction of the valve assembly, first stage and second stage disc valves are arranged in series or in tandem fashion with respect to a fluid path for fluid communication between upper and lower working chambers defined in a shock absorber cylinder. The first stage valve is designed to respond to the lower pressure to be exerted thereonto to open for fluid communication therethrough. On the other hand, the second stage valve is designed to respond to the higher pressure than the first stage valve to open for fluid communication therethrough. The second stage valve also defines a constant orifice or orifices having a predetermined fixed fluid path area for constantly permitting fluid flow therethrough in a limited flow rate.
With the construction set forth above, in a response to the low speed piston stroke which creates a smaller pressure difference between the upper and lower working chamber and thus small pressure to exert on the first stage and second stage valves. The first stage valve is responsive to this small pressure to establish fluid communication between the upper and lower fluid chambers. Therefore, the working fluid flows through the gap formed in the first stage valve and the constant orifice. In such case, since the path area is limited to be small, substantially great flow restriction for the working fluid is provided to generate great damping force to damp the vibration induced in the vehicle body. On the other hand, in response to high frequency piston stroke which creates greater pressure difference between the upper and lower working chambers, both first stage and second stage valves are open to provide increased fluid path area to produce smaller damping force. Therefore, the vibration energy input from the road wheel can be absorbed to avoid a rough ride.
In such a valve construction, the first stage valve is driven to deform at substantially higher frequency than that of the second stage valve. Therefore, the first stage valve may have shorter life in comparison with that of the second stage valve. Particularly, when the first stage valve has constant resilient characteristics, it may be subject substantially high stress to further shorten the life.
On the other hand, at a pressure difference greater than the pressure relief point of the second stage valve, the damping characteristics at a flow restriction orifice extending through the shock absorber piston varies at a rate proportional to two power of the piston stroke speed. On the other hand, at the same time, the damping characteristics at the first and second stage valves varies at a rate proportional to two over three power of the piston stroke speed. As a result, variation characteristics of overall damping characteristics of the shock absorber cannot be linear to cause difficulty of setting desired variation characteristics.